DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
685 
frontal. The greater part of this bone consists of an expansion of the upper layer 
only. The lower layer is developed, but to a very small degree, and that chiefly in 
the central portions of the bone. In some parts of the periphery, the thin mem- 
braniform structure appears to rest, not upon its own proper cartilaginous matrix, 
but upon that of one of the contiguous subjacent bones. 
It will not be necessary to pursue the details of this portion of the Pike’s skeleton 
any further. The examples already given suffice to show that all the bones entering 
into the composition of its cranium, have been produced according to one common 
plan, which, with various minor modifications, is identical with that on which the 
less complex carpal bone was constructed. There are, however, some other examples 
requiring a more special notice. 
In the very young Pike, a small cylindrical rod of cartilage extends through the 
dentary, from the symphysis of the lower jaw, to the angle where it receives the 
anterior extremity of the mandible, and which is even prolonged a considerable way 
across the inner surface of the latter bone. This small cartilaginous rod has been 
the primary matrix of the dentary. In older fish its anterior extremity appears to 
become absorbed, the cavity being occupied by secondary cancelli, containing adi- 
pose tissue. The rest of the bone has been of primary intra-rnembranous growth. 
The premaxillary exhibits similar features. It contains a central oval rod of car- 
tilage, around which are arranged the investing layers of membraniform bone. In 
the latter portions there are some large cavities, which appear to have been formed, 
not by absorption of the cartilage, but by inflections of the membranous lamellm 
prior to their calcifieation. In the dental surface of this bone, as seen in a transverse 
vertical section, we observe a curious arrangement of the Haversian canals. Imme- 
diately beneath the base of eaeh tooth, there exists a group of small anastomosing 
canals, scarcely distinguishable from those seen in the contiguous portion of the tooth 
itself, and into which they open. Bearing in mind the intra-membranous origin of 
this portion of the bone, the above fact has a significant bearing upon the origin of the 
teeth. The central laminae of the premaxillary contain a remarkable series of very 
minute parallel canals or tubes, which pass obliquely through them. 
The presphenoid and vomer exhibit a peculiarity of structure, which at first sight 
appears to distinguish them from the other bones which I have examined ; but this 
difference is more apparent than real. A thick cartilage intervenes between the 
vomer and the nasal bone, and a prolongation of the same cartilage runs backwards 
along the upper surface of the presphenoid. When vertical sections are made of the 
latter of these bones, we find that it appears to be developed on the inferior surface 
of the cartilage. The new lamellae, which are membraniform, are added partly to 
the inferior surface of those pre-existing, but chiefly to their superior one, where a 
thin space, apparently lined with membrane, separates the bone from the contiguous 
cartilage. The inferior surface of the latter has not only a broad longitudinal depres - 
sion receiving the upper surface of the bone, but also a long narrow central groove, 
MDCCCLI. 4 T 
